Microstructural features, diffusion and molecular relaxations in polyimide/silica hybrids

FTIR spectroscopy has been used to investigate the microstructure of the inorganic phase in polyimide/silica hybrids obtained by the sol-gel route. It has been shown that the presence of a coupling agent (GOTMS) strongly influences the silica phase by favoring the formation of linear, branched chains which make the structure loosely interconnected. The morphological changes induced by GOTMS are reflected in the water sorption properties of the investigated systems. Due to the hydrophilic nature of the silica phase, the amount of sorbed water increases in the hybrid systems in comparison to the pristine polyimide, and among the two hybrids, increases more in the two-phase system than in the co-continuous. Molecular interactions of the hydrogen-bonding type have been identified between the imide units and the H2O molecules, both in the polyimide and in the hybrid systems. Finally, the sub-Tg relaxation processes have been investigated in detail by dynamic-mechanical analysis coupled with molecular mechanics simulations. Considerable effects of the inorganic phase on these processes were observed, especially for the case of the co-continuous systems, and were accounted for by molecular scale contiguity among the phases.